This invention relates to E.C.C.M. methods and apparatus and more particularly to methods and apparatus for rendering ineffective D.E.C.M. apparatus carried by aircraft and intended to present a false target to tracking radar.
Tracking radars are widely employed, for example at S.A.M. (surface-to-air missile) sites. These radars include automatic ranging circuitry having the capability of "locking on" a designated target after it has been acquired and is being tracked by the radar operator. The ranging circuitry includes error detector means which provides tracking error data as an output. The error data is combined with range data derived from the operator's controls to provide corrected range data which, together with azimuth and elevation data from the radars, can be utilized to guide a S.A.M. accurately to the tracked target. Generally, the error detector means detects shifts in the power of the target video signals along a time reference base and causes a range gate to continually tend to be centered with respect to the target return. In some radars the error detector means utilizes an early gate and a late gate to detect the direction and extent of shift of the target video. In other radars, such as that designated the MSQ-44, the target video is presented in the detector twice, once as early and once as late target video, the early and late video being compared against the range gate which, as before tends to be centered with respect to the target video. The foregoing is generally applicable to conventional tracking radars as well as to more modern TWS (track-while-scan) radars. One distinguishing feature of the TWS radar which the invention takes into account is the fact that TWS radar does not have true target video available at times when the antenna scan is not directed substantially at the target.
Now, in order to avoid the loss of aircraft to enemy S.A.M. action guided by radars having automatic tracking circuitry, it has become the practice to equip probable target aircraft with D.E.C.M. equipment which serves to break the lock on the true target. The D.E.C.M. accomplishes this by providing a stronger false target return which initially at least partially overlaps the true target return. The false return is thereafter progressively delayed according to a predetermined program so that the false target video "walks away" from the true target video. Because the false target has greater power, the error detector means causes the range gate to follow and center on the false target return thereby breaking the lock on the true target and resulting in erroneous range data. Moreover, by further modulating the false return, the D.E.C.M. can cause deceptive azimuth and elevation data to be rendered by the tracking radar.
Although the TWS has target video only when the antenna main lobe is directed toward the target, the D.E.C.M. provides false returns at other times generated from the radar side lobes. These false returns are also effective in breaking the lock on the true target.